WO2017008371A1

WO2017008371A1 - Method for predicting flammable range of thermodynamic cycle mixed working medium

Info

Publication number: WO2017008371A1
Application number: PCT/CN2015/087986
Authority: WO
Inventors: 李家俊
Original assignee: 天津大学
Priority date: 2015-07-13
Filing date: 2015-08-25
Publication date: 2017-01-19
Also published as: CN105117786B; CN105117786A

Abstract

Provided is a method for predicting the flammable range of a thermodynamic cycle mixed working medium. The method predicts the upper flammable limit and the lower flammable limit of a mixed working medium respectively, and comprises establishing a flammable limit prediction model based on a critical flame temperature theory, then carrying out linear fitting on a flame limit value of an HC_s/CO₂ mixed working medium obtained from a test and a corresponding CO₂ volume fraction according to the predication model, and obtaining an undetermined coefficient in the predication model so as to obtain an optimal flammable limit prediction model. The method overcomes the defect that a theoretical prediction model needs to depend on the selection of an appropriate critical flame temperature to be determined, greatly improves the predication precision, ensures the accuracy of the flammable limit predication data of the HC_s/CO₂ mixed working medium, and provides an accurate reference basis for the safety assessment in the case where this type of mixed working medium is applied to a thermodynamic cycle system.

Description

Prediction method for combustible range of thermodynamic circulating mixed working fluid

Technical field

The invention belongs to the technical field of thermodynamic cycles, and particularly relates to a method for predicting the combustible range of a thermodynamic circulating mixed working fluid.

Background technique

In recent years, the problems of energy shortage and environmental pollution have become more and more serious. Carrying out energy conservation and emission reduction is an effective way to alleviate these two major problems. The research on power generation technology such as waste heat (such as exhaust heat and industrial waste heat of internal combustion engines) is of great significance for reducing fossil energy consumption and greenhouse gas emissions such as CO ₂ . The use of organic Rankine cycle for waste heat power generation is an effective way to save energy and reduce emissions. However, if the residual heat temperature is too high than the decomposition temperature of the organic working fluid, the thermal properties of the working fluid will change, which will affect the performance of the entire circulation system. . Therefore, for the use of medium and high temperature waste heat with high temperature exhaust characteristics such as internal combustion engines, there is still a lack of suitable circulating working fluid. The medium-high temperature mixed working fluid with good cycle performance is mainly concentrated in hydrocarbon HCs and siloxanes, but they are flammable and explosive dangerous substances. Once the leakage occurs, the consequences are unimaginable, which limits further promotion and practical application.

The safety and environmental protection flame retardant CO ₂ is added to the hydrocarbon working fluid to form a mixed working medium, and the properties thereof can be adjusted according to the composition ratio of the components, thereby fully suppressing the aforementioned flammability and improving the cycle performance. . In addition, both HCs and CO ₂ are natural working fluids, ODP values are equal to zero, GWP values are small, in line with the development of environmentally friendly working fluids, and have the potential to become a long-term alternative working medium in the field of medium and high temperature waste heat utilization. However, the flammability range of this type of mixed working fluid is very important for the safety assessment of its practical application under different mixing ratios.

The flammable range refers to the upper and lower limits of combustion that can be produced by the working fluid. At present, the Le Chatelier empirical formula and the critical flame temperature method are the most common methods for predicting the flammability limit of mixtures. The Le Chatelier empirical formula mainly predicts the flammability limit of combustible working fluids composed of a mixture of multiple combustible components. However, the flammability limit prediction accuracy of a mixture containing non-combustible substances similar to HCs/CO _{2 is} low. The method of critical flame temperature theory is used to predict the high flammability limit of HCs/CO ₂ mixture, but how to determine its relatively accurate critical temperature is very difficult. If the problem is solved then the method can be applied to mixtures of different ratios and compositions.

Summary of the invention

In order to solve the above problems, it is an object of the present invention to provide a method for predicting the combustible range of a thermodynamic circulating working fluid composed of HCs/CO ₂ .

Based on the theory of critical flame temperature, the method establishes the HCS/CO ₂ flammability limit prediction model for mixed working fluids. By linearly fitting the flammability limit values of five or more mixed working fluids and the corresponding CO ₂ volume fractions, the prediction model is obtained. Undetermined coefficient. This coefficient is verified by a large amount of experimental data, so that the accuracy of the flammability limit prediction of the HCs/CO ₂ mixture is greatly improved.

In order to achieve the above object, the technical solution of the present invention is as follows: The method for predicting the combustible range of the thermodynamic circulating mixed medium predicts the upper flammable upper limit and the lower flammable lower limit of the binary mixed working medium composed of HCs/CO ₂ .

(1) The prediction methods for the HCs/CO ₂ flammable upper limit include:

1.1 Establish a flammable upper limit prediction model for HCs/CO ₂ mixed working fluid:

Where U _m is the upper limit of flammability of the HCs/CO ₂ mixture; U ₀ is the upper limit of flammability of the corresponding hydrocarbon substance; y is the volume fraction of CO ₂ in the HCs/CO ₂ mixture; K _U is the undetermined coefficient of the flammable upper limit prediction model, This coefficient is related to the selected critical flame temperature.

1.2 Test the upper limit of flammability of HCs/CO ₂ mixture containing different volume fractions of CO ₂ , and obtain the upper limit of flammability of mixed working fluids with 5 or more CO ₂ volume fractions;

1.3 The flammable upper limit obtained in step 1.2 and the corresponding CO ₂ volume fraction are fitted according to the prediction model A, and the undetermined coefficient K _U in the prediction model A is obtained;

1.4 The flammable upper limit of the HCs/CO ₂ mixed working fluid containing any CO ₂ volume fraction is predicted based on the undetermined coefficient K _U .

(2) The prediction methods for the lower flammability limit of HCs/CO ₂ include:

2.1 Establish a flammable lower limit prediction model for HCs/CO ₂ mixed working fluid:

Where L _m is the lower flammable lower limit of the HCs/CO ₂ mixture; L ₀ is the lower flammable lower limit of the corresponding hydrocarbon substance; y is the volume fraction of CO ₂ in the HCs/CO ₂ mixture; K _L is the undeterminable coefficient of the flammable lower limit prediction model, This coefficient is also related to the selected critical flame temperature.

2.2 The lower flammability limit of the HCs/CO ₂ mixture containing different volume fractions of CO ₂ is tested, and the lower flammable lower limit values of the mixed working fluids with different CO ₂ volume fractions are obtained;

2.3 Fitting the flammable lower limit value obtained in step 2.2 with the corresponding CO ₂ volume fraction according to the prediction model B, and obtaining the undetermined coefficient K _L in the prediction model B;

2.4 The flammable lower limit value of the HCs/CO ₂ mixed working fluid containing any CO ₂ volume fraction is predicted based on the undetermined coefficient K _L .

The advantages and benefits of the method of the present invention are: based on the critical flame temperature theory, the two prediction models of the upper and lower flammability limits are established, and the flammability limit can be predicted for any volume fraction mixture composed of HCs/CO ₂ . The undetermined coefficients in the prediction model obtained by fitting are verified by experimental data, which overcomes the defects that the theoretical prediction model needs to rely on selecting the appropriate critical flame temperature, which greatly improves the prediction accuracy. The prediction accuracy can be controlled within 5%, which ensures the accuracy of the HCs/CO ₂ mixed working fluid flammability limit prediction data, and provides an accurate reference for the safety assessment of the thermal circulation system using this type of mixed working fluid.

detailed description

The method and technical solution of the present invention will be described in detail below through specific embodiments. It should be noted that the described embodiments are only used for a thorough explanation of the method of the invention. Based on the basic principles and processes of this embodiment, other data implementations that have not been involved are within the scope of the present invention.

A method for predicting the upper and lower flammability limits of a binary mixed working fluid consisting of HCs/CO ₂ includes the following steps:

(1) The prediction methods for the HCs/CO ₂ flammable upper limit include:

1.1 Based on the critical flame temperature theory, establish a flammable upper limit prediction model for HCs/CO ₂ mixed working fluid:

1.2 According to ASTM (American Society for Testing and Materials) E681:04 standard, the upper limit of flammability of HCs/CO ₂ mixture containing different volume fractions of CO ₂ is tested, and more than 5 sets of mixed CO ₂ volume fractions are obtained. Limit.

1.3 The flammable upper limit obtained in step 1.2 and the corresponding CO ₂ volume fraction are fitted according to the prediction model A to obtain the undetermined coefficient K _U in the prediction model A.

After the undetermined coefficient is determined, Lm and Um are in a one-to-one relationship with y, that is, as long as the volume fraction of CO ₂ is known, the combustible range of the mixed working medium can be predicted.

Taking propane/CO ₂ and isobutane/CO ₂ as examples, Table 1 gives the prediction effect of the prediction method on the practical application of the upper limit of flammability.

2.1 Based on the critical flame temperature theory, establish a flammable lower limit prediction model for HCs/CO ₂ mixed working fluid:

Where L _m is the lower flammable lower limit of the HCs/CO ₂ mixture; L ₀ is the lower flammable lower limit of the corresponding hydrocarbon substance; y is the volume fraction of CO ₂ in the HCs/CO ₂ mixture; K _L is the undeterminable coefficient of the flammable lower limit prediction model, The coefficient is also related to the selected critical flame temperature;

2.2 According to the ASTM (American Society for Testing and Materials) E681:04 standard, the lower flammable lower limit of HCs/CO ₂ mixture containing different volume fractions of CO ₂ is obtained, and the mixed working fluids of 5 or more different CO ₂ volume fractions are obtained. Limit value

Taking propane/CO ₂ and isobutane/CO ₂ as examples, Table 2 gives the prediction effect of the prediction method on the practical application of the lower flammable limit.

The flammability limit data in steps 1.2 and 2.2 is the actual data obtained from the actual test.

When the undetermined coefficient in the prediction model is determined, the HCs/CO ₂ mixed working flammability limit of any CO ₂ volume fraction can be predicted. The error of the combustible range of the mixed working fluid calculated by the prediction model is within 5%.

Claims

A method for predicting a flammable range of a thermodynamic circulating mixed working fluid, characterized in that a method for predicting a flammable upper limit and a lower flammable lower limit of a binary mixed working medium composed of HCs/CO 2 is predicted, comprising the following steps:

(1) The prediction methods for the HCs/CO 2 flammable upper limit include:

1.1 Establish a flammable upper limit prediction model for HCs/CO 2 mixed working fluid:

Where U m is the upper limit of flammability of the HCs/CO 2 mixture; U 0 is the upper limit of flammability of the corresponding hydrocarbon substance; y is the volume fraction of CO 2 in the HCs/CO 2 mixture; K U is the undetermined coefficient of the flammable upper limit prediction model, The coefficient is related to the selected critical flame temperature;

1.2 Test the upper limit of flammability of HCs/CO 2 mixture containing different volume fractions of CO 2 , and obtain the upper limit of flammability of mixed working fluids with 5 or more CO 2 volume fractions;

1.3 The flammable upper limit obtained in step 1.2 and the corresponding CO 2 volume fraction are fitted according to the prediction model A, and the undetermined coefficient K U in the prediction model A is obtained;

1.4 predicting the flammable upper limit of HCs/CO 2 mixed working fluid containing any CO 2 volume fraction according to the undetermined coefficient K U ;

(2) The prediction methods for the lower flammability limit of HCs/CO 2 include:

2.1 Establish a flammable lower limit prediction model for HCs/CO 2 mixed working fluids;

Where L m is the lower flammable lower limit of the HCs/CO 2 mixture; L 0 is the lower flammable lower limit of the corresponding hydrocarbon substance; y is the volume fraction of CO 2 in the HCs/CO 2 mixture; K L is the undeterminable coefficient of the flammable lower limit prediction model, The coefficient is related to the selected critical flame temperature;

2.2 The lower flammability limit of the HCs/CO 2 mixture containing different volume fractions of CO 2 is tested, and the lower flammable lower limit values of the mixed working fluids with different CO 2 volume fractions are obtained;

2.3 Fitting the flammable lower limit value obtained in step 2.2 with the corresponding CO 2 volume fraction according to the prediction model B, and obtaining the undetermined coefficient K L in the prediction model B;

2.4 The flammable lower limit value of the HCs/CO 2 mixed working fluid containing any CO 2 volume fraction is predicted based on the undetermined coefficient K L .